1. Field of the Invention
The present invention relates to a capacitance detecting circuit of a capacitance detecting type sensor such as a pressure sensor, an acceleration sensor, and an angular velocity sensor utilized in vibration measurements, vehicle controls, and motion controls.
2. Description of the Related Art
Very recently, specific attentions are paid to inertia sensors capable of detecting pressure of fluids, and pressure, acceleration, or angular velocity applied to moving objects, in particular to such inertia sensors which utilize the micromachining technique of semiconductor industries and may detect measurement signals by detecting changes in capacitances of a capacitor. These sensors own merits, for instance, a compactness of an apparatus, mas-producibility, high precision, and high reliability.
FIG. 13 is a sectional structural diagram for showing a typical capacitance type acceleration sensor which is manufactured by employing the micromachining process of semiconductor. This sensor owns a structure such that a silicon mass member 1 is supported through a beam 3 by an anchor portion 2. Fixed electrodes 4 and 5 are formed above/beneath this mass member 1 on glass, or silicon 6. The mass member 1 and the fixed electrodes 4, 5 constitute capacitors 7 and 8 shown in FIG. 14. These capacitors 7 and 8 may constitute a sensor element 9.
When inertia force caused by acceleration is exerted on the mass member 1 along an x direction, the mass member 1 is displaced. Due to this displacement, one capacitance value between the mass member 1 and the fixed electrodes 4, 5 is increased (C+.DELTA.C), and the other capacitance value between them is decreased (C-.DELTA.C). A change in the capacitance values is converted into a voltage output.
As the method for converting the change in the capacitance values in response to the displacement of the mass member 1 into the voltage output, for instance, an example of the interface circuit for utilizing the switched capacitor circuit is described in the publication written by H.LEUTHOLD and F.RUDOLF, An ASIC for High-resolution Capacitive Microaccelerometers, Sensors and Actuators, A21-A23, 1990, pages 278 to 281.
FIG. 15 is a circuit diagram for representing an example of a capacitance type sensor interface circuit to which the above-described conventional switched capacitor circuit is applied. FIG. 16 shows timing of clock signals .phi.1 and .phi.2 for the respective switches indicated in FIG. 15. The clock signals .phi.1 and .phi.2 alternately become ON (High). In order that these clock signals are not turned ON together, a common OFF (Low) time period is provided.
At the timing of the clock signals .phi.1, the power source voltage Vs and the ground (Gnd) are connected to both terminals of the sensor element 9, and at this time, an error charge .DELTA.Q corresponding to a difference in the capacitances of the capacitors C1 and C2 is sampled by a switched capacitor circuit 10 connected subsequent to this sensor element 9. As a result, an error voltage Vm (=Vout-Vr) in response to the error charge .DELTA.Q is produced, this error voltage is held in the capacitor C5 at the timing of the clock signal .phi.2 through a voltage hold/feedback circuit 11 connected subsequent to this switched capacitor circuit 10, and further, this error voltage Vm is fed back to the capacitor C6. Accordingly, the potential at the non-inverting input of a first-staged OP amplifier for constituting the switched capacitor circuit 10 becomes higher/lower than the reference voltage Vr in response to the symbol of the error charge .DELTA.Q. This error voltage Vm is changed in stepwise every switching cycle, and then becomes a constant value expressed by the below-mentioned formula (1) at a time instant when the error charge .DELTA.Q becomes zero, namely being brought into such a condition that the same charges are accumulated into the capacitor C1 and the capacitor C2 every time.
For the sake of convenience, assuming now that an input offset voltage is commonly used for the first-staged OP amplifier and the second-staged OP amplifier, namely is set to "Vos", and also Vr=Vs/2, the error voltage is given by: ##EQU1##
In this formula, symbol S=(C1-C2)/(C1+C2) indicates an index of a sensor sensitivity, and an essential sensitivity becomes S/2.
In the above case, it is assumed that the capacitance type sensor interface circuit is driven under single power source Vs. Alternatively, assuming now that the capacitance type sensor interface circuit is driven under positive/negative 2 power sources of .+-.Vs/2, and also Vr is equal to a zero potential, if the formula (1) is rewritten, then the following formula (2) is given as follows: ##EQU2##
As indicated in the formula (1) or (2), the output voltage Vout may be expressed by a summation of the DC offset voltages in correspondence with the output voltage corresponding to the capacitance difference produced in response to the displacement of the mass member 1 caused by the acceleration, and the input offset voltage Vos of the OP amplifier.
On the other hand, since the error voltage Vm is fed back, it is required to satisfy the formula (3) as a stability condition.
In this formula, symbol "Co" is equal to initial capacitances (C1=C2=Co) of the capacitor C1 and the capacitor C2 when the capacitance difference (C1-C2) caused by the displacement of the mass member 1 in response to the acceleration becomes zero. EQU Co/{1-[(C1-C2)/(C1+C2)].sup.2 }.times.C4/(C3.times.C5)&lt;1 (3)
As described above, in accordance with the conventional method, C3 must be set to be small and Co must be set to be large in order to reduce the DC offset voltage. However, when C3 is set to be small, there is a drawback that the stable characteristic (converging characteristic) expressed by the formula (3) would be deteriorated.
Also, since the offset output voltages Vos of the OP amplifiers are fluctuated, depending upon the respective OP amplifiers, and are varied in response to the temperatures, the DC offset voltage expressed by the formula (1) is similarly fluctuated and thus represents the temperature depending characteristic.
Since the conventional capacitance detecting circuit is constructed by employing the above-described arrangement, the four capacitors (C3 to C6) are required as the passive element, and at least two sets of OP amplifiers are required as the active element in the capacitance type sensor interface circuit arrangement shown in FIG. 15. As a result, the area of the semiconductor IC circuit would be increased, and thus the dimension of the IC chip would be increased. There is a problem that the cost of the circuitry IC would become higher.
Also, when a plurality of sensor elements are employed, since the same detecting circuits must be provided with the respective capacitance type sensors, the detecting circuits whose quantity is equal to that of the sensors are required. As a consequence, there is another problem that the entire circuit can be hardly made compact.